This invention relates to the field of apparatus for co-extruding laminated polymer films or sheets, and more particularly to co-extrusion apparatus adapted to selectively deliver polymer from a plurality of extruders to reconfigurable, substantially parallel, converging channels so as to form laminated plastic films or sheets.
Plug-type flow diverters or layer sequencers have been employed in the co-extrusion of laminated plastic films and sheets. Known plug-type layer sequencers, such as those taught in Cloeren U.S. Pat. No. 4,839,131 and Cloeren et al. U.S. Pat. No. 4,784,815, comprise housings and cylindrical selector plugs or pins which cooperate to direct resin or fluent polymer from a plurality of extruders into sequences of polymer flows which can be recombined to form the films or sheets. One advantage to the use of plug-type layer sequencers is adaptability: a single layer sequencer may be used to produce a number of different sequences of polymer flows and, hence, a number of different laminated structures, merely by substituting selector pins having different arrangements of channels for distributing the polymer flows. Another advantage is that the selector pins can be removed for cleaning. Once installed, however, a selector pin must form a tight seal with the housing so as to confine the polymer flows, which can reach pressures as high as 4.5 kpsi and temperatures as high as 640° F. (˜338° C.), in the channels.
One drawback to known plug-type layer sequencers is the difficulty of removing the selector pin for cleaning or substitution without causing galling or other mechanical damage to the associated surfaces of the selector pin and the housing. In order to minimize such damage, the clearances between the associated surfaces of the selector pins and the housings of known layer sequencers are generally around 0.008 inch to 0.012 inch (˜0.2 mm to ˜0.3 mm). Such clearances are larger than desirable for forming metal-to-metal mechanical seals. As a consequence, it is possible for some polymer to flow into the spaces between the selector pins and the sockets, and for the polymer flowing into these spaces to degrade or char, making the removal of the selector pins difficult. There remains a need in the art for co-extrusion apparatus including plug-type layer sequencers having readily demountable selector pins nonetheless capable of forming tight mechanical seals with associated housings.
In conventional co-extrusion apparatus, differences in rheology and flow geometry between adjacent polymer flows can produce shear forces and other disuniformities between the adjacent flows, resulting in distortion of the layers of a film or sheet produced by the apparatus. Although these shear stresses can be reduced to a greater or lesser degree by placing the layer sequencers of the assemblies close to the combining blocks in which the polymer flows are combined to form the films or sheets, other engineering considerations often require substantial spacings between the layer sequencers and the combining blocks.
A further drawback of prior art designs resides in a difficulty of maintaining very thin skin layers. Layer uniformity with poor accuracy limits the ability to extrude thin layers of polymers with improved properties that can yield significant cost savings. Further difficulties with many conventional combining blocks include complexity, relatively high expense and lack of an ability to provide individual select flow paths and geometries without changing out and replacing the entire blocks.
For some applications, it would be desirable to produce laminated plastic films or sheets having layers or strata including sharply-defined lanes of differing polymer composition. There remains a need in the art for co-extrusion-apparatus capable of producing films or sheets having such lanes.